1. Field of the Invention
The present invention relates to an image sensor made in monolithic form intended to be used in shooting devices such as, for example, film cameras, camcorders, cell phones, or again digital photographic cameras.
2. Discussion of the Related Art
FIG. 1 schematically illustrates an example of a circuit of a photosensitive cell of an array of photosensitive cells of an image sensor. With each photosensitive cell of the array are associated a precharge device and a read device. The precharge device is formed of an N-channel MOS transistor M1, interposed between a supply rail Vdd and a read node S. The gate of precharge transistor M1 is capable of receiving a precharge control signal RST. The read device is formed of the series connection of first and second N-channel MOS transistors M2, M3. The drain of first read transistor M2 is connected to supply rail Vdd. The source of second read transistor M3 is connected to an input terminal P of a processing circuit (not shown). The gate of first read transistor M2 is connected to read node S. The gate of second read transistor M3 is capable of receiving a read signal RD. The photosensitive cell comprises a photodiode D having its anode connected to a reference supply source GND, for example, the circuit ground, and having its cathode connected to node S via an N-channel charge transfer MOS transistor M4. The gate of transfer transistor M4 is capable of receiving a charge transfer control signal T. Generally, signals RD, RST, and T are provided by control circuits, not shown in FIG. 1, and may be provided to all the photosensitive cells of a same row of the cell array. Node S behaves a as charge storage region, the apparent capacitance at read node S being formed of the source capacitances of transistors M1 and M4, of the input capacitance of transistor M2, as well as of all the stray capacitances present at node S. According to a variation, a specific diode having its cathode connected to node S and having its anode connected to ground may be provided.
The operation of this circuit will now be described. A photodetection cycle starts with a precharge phase during which a reference voltage level is imposed at read node S. This precharge is performed by turning on precharge transistor M1. Once the precharge has been performed, precharge transistor M1 is turned off. The reference charge state at node S is then read. The cycle carries on with a transfer to node S of the photogenerated charges, that is, those created and stored in the presence of radiation, in photodiode D. This transfer is performed by turning on transfer transistor M4. Once the transfer is over, transistor M4 is turned off, and photodiode D starts photogenerating and storing charges which will be subsequently transferred to node S. Simultaneously, at the end of the transfer, the new charge state at node S is read. The output signal transmitted to terminal P then depends on the channel pinch of first read transistor M2, which is a direct function of the charge stored in the photodiode.
FIG. 2 shows a simplified top view of an image sensor made in monolithic form. FIG. 2 illustrates a conventional example of distribution of the electronic components (photodiodes and transistors) associated with the image sensor. The transistors and the photodiodes associated with the photosensitive cells are generally formed at the center of the image sensor at the level of block 1 (pixels). The transistors of the peripheral circuits which, generally, carry out various processings of the signals associated with the photosensitive cells, are formed all around block 1. As an example, blocks 2 (readout) correspond to the circuits dedicated to the provision of the control signals of the array of photosensitive cells and to the reading of the signals provided by the photosensitive cells (especially the previously-mentioned processing circuits). Generally, other peripheral circuits may be provided to perform additional functions directly at the level of the image sensor, such as, for example, the correction of faults of the signals read from the read nodes of the photosensitive cells, the image storage, signal processing operations, etc. Thus, block 3 (memory) may correspond to peripheral circuits dedicated to the storage of images. Blocks 4 (digital) may correspond to peripheral circuits dedicated to the performing of signal processing operations. Blocks 5 may correspond to the peripheral circuits dedicated to the processing of input/output interface signals, and especially comprise transistors which are directly connected to the connection pads of the image sensor.
Conventionally, the electronic components of the image sensor are formed at the level of a substrate of a semiconductor material, for example, a silicon wafer, covered with a stack of insulating layers at the level of which are formed the conductive tracks and vias enabling connection of the electronic components of the image sensor. The stack of insulating layers is covered, at least at its central portion, with colored filters and lenses associated with the photosensitive cells, with the possibility for the colored filters not to be present when the image sensor is a black and white sensor. Such an image sensor is said to be front-lit.
A disadvantage of a front-lit image sensor is that the straight path of the light rays from each lens to the photodiode of the associated photosensitive cell may be hindered by the tracks and the conductive vias present at the level of the insulating layer stack covering the substrate. It may then be necessary to provide additional optical devices, in addition to the previously-mentioned lenses, to make sure that most of the light rays which reach the front surface of the image sensor reach the photodiodes of the photosensitive cells. This then results in image sensors that may have a relatively complex structure, difficult to form.
A solution to avoid the use of additional optical devices and/or to improve the light absorption at the level of the image sensor substrate comprises lighting the image sensor through the rear surface of the substrate at the level of which the photodiodes are formed. The image sensor is said to be back-lit.
FIG. 3 shows an example of conventional monolithic forming of a back-lit image sensor. In the right-hand portion, the photodiode D and the transistor M4 of a photosensitive cell of the image sensor have been shown and, in the left-hand portion, two MOS transistors M5 and M6 associated with the peripheral circuits of the image sensor have been shown. The image sensor comprises a lightly-doped P-type substrate 14 (P−) comprising a front surface 15 and a rear surface 16. The photosensitive cell and the transistors of the peripheral circuits are, as an example, delimited by field insulation regions 20, for example, made of silicon oxide, each surrounded with a P-type region 22 more heavily doped than substrate 14 (P+). Photodiode D comprises an N-type region 24 formed in substrate 14. In the case where photodiodes of fully depleted type are used, region 24 is covered with a P-type region 26 more heavily doped than substrate 14. An N-type region 28, formed in substrate 14, corresponds to the drain region of transistor M4. An insulating region 30 extends on front surface 16 of substrate 14, between regions 28 and 24 and corresponds to the gate oxide of transistor M4. Insulating portion 30 is covered with a polysilicon portion 32 corresponding to the gate of transistor M4. A P-type well 33, formed in substrate 14, more heavily doped than substrate 14 (P+), corresponds to the well of transistor M5. Two N-type regions 34, formed in well 33, correspond to the power terminals of transistor M5. An insulating portion 35 extends between regions 34 and corresponds to the gate oxide of transistor M5. A polysilicon portion 36 covers insulating portion 35 and corresponds to the gate of transistor M5. An N-type well 37, forming substrate 14, corresponds to the well of transistor M6. Two P-type regions 38, formed in well 37, correspond to the power terminals of transistor M6. An insulating portion 39 extends between regions 38 and corresponds to the gate oxide of transistor M6. A polysilicon portion 40 covers insulating region 39 and corresponds to the gate of transistor M6.
Substrate 14 is covered with a stack of insulating layers 41 at the level of which are formed metal tracks 44 of different metallization levels and metal vias 46 enabling connection of the components of the photosensitive cells and of the peripheral circuits. Stack 41 is covered with an insulating layer 42. A reinforcement 43, for example corresponding to a solid silicon wafer, covers insulating layer 42. A P-type implantation 44, more heavily doped than the substrate, is formed of the side of rear surface 16 of substrate 14. When the image sensor is a color sensor, a colored filter 48 covered with a lens 50 on the side of rear surface 16 of substrate 14 is provided. At the level of the peripheral circuits, an insulating layer 52 covers rear surface 16 of substrate 14.
A back-lit image sensor has the advantage that the path of the light rays which reach the sensor on the side of rear surface 16 is not hindered by metal tracks and vias 44, 46 provided at the level of insulating layer stack 41.
Among the peripheral circuits, some exhibit a significant heat dissipation. This concerns, for example, power supply generation circuits, high-frequency output stages, phase-locked loops, etc. A disadvantage is that an image sensor is very sensitive to temperature. Indeed, the operating principle of the image sensor corresponds to the absorption of photons in substrate 14, which causes the generation of electron/hole pairs, the electrons being captured by the photodiodes of the photosensitive cells. However, thermal electrons are also capable of being captured by the photodiodes. This translates as the occurrence of a thermal noise at the level of the signals measured from the read node of a photosensitive cell which is generally called “dark current”. When present, it is preferable that the dark current be substantially identical for all the photosensitive cells of the image sensor so that the signals measured at the read nodes, in particular in case of a low lighting, have a substantially uniform amplitude. It is thus desirable for the substrate in which the photodiodes of the photosensitive cells are formed to be maintained at as uniform a temperature as possible and, if possible, at a temperature which remains moderate.
When the image sensor is front-lit, the substrate in which the electronic components of the image sensor are formed generally has a thickness of several hundreds of micrometers. Such a substrate enables a good carrying off of the heat generated by high thermal dissipation circuits. Further, the substrate is generally arranged at the level of a thermally conductive package further easing the heat carrying-off. Thereby, the substrate temperature remains substantially uniform, which enables keeping a relatively constant dark current, when present, through all the photosensitive cells.
A difficulty appears when the image sensor is back-lit since substrate 14 then has a low thickness, for example, on the order of a few micrometers, and is thermally isolated. It is then difficult to carry off the heat generated by peripheral circuits with a significant heat dissipation. This translates as local temperature variations that may cause a local increase in the dark current.